In vitro enhancement of bone marrow natural killer cells after incubation with thymopoietin32-36 (TP-5).

Natural killer (NK) activity and interferon (IFN) production of spleen and bone-marrow lymphocytes were investigated in 8 to 10-wk-old C3HeB/FeJ/Cne mice, before and after in vitro exposure to the synthetic pentapeptide corresponding to positions 32-36 of the sequence of thymopoietin (TP-5). NK activity was studied against the 51Cr-radiolabelled Moloney virus-induced lymphoid cell line YAC 1, while IFN titres were determined by the inhibition of cytopathology of vesicular stomatitis virus on L929 cells, scored for CPE after 24 h from viral infection. A part of the lymphocytes from the spleen and bone marrow were incubated with different preselected doses (0.1, 1 and 10 micrograms/ml) of TP-5 for 1 h at 37 degrees C, and a part were left unincubated. At the end of the incubation time they were tested, without being washed, for NK activity and IFN production. TP-5 was able to significantly enhance (P less than 0.001 chi 2 test with Yate's correction) bone-marrow NK activity at a dose of 1 microgram/ml, while it was ineffective on spleen cells. A decrease of NK activity, seemingly due to a toxic effect, was observed both in bone-marrow and spleen lymphocytes, after incubation with a dose of 10 micrograms/ml. IFN production was not enhanced after exposure to TP-5. In all, our experiments suggest that TP-5 may enhance bone-marrow NK cells, perhaps by permitting the maturation of their precursors, as already known for T-cell-mediated cytotoxicity. Its effect is independent on IFN production and might be able to induce the rearrangement of certain surface specific receptors.